Transcriptional down-regulation of poly(ADP-ribose) polymerase gene expression by E1A binding to pRb proteins protects murine keratinocytes from radiation-induced apoptosis.

Adenovirus E1A confers enhanced cell sensitivity to radiation and drug-induced DNA damage by a mechanism involving the binding to cellular proteins. Mutant analysis in E1A-transfected murine keratinocytes demonstrates that increased sensitivity to DNA damage requires at least E1A binding to the p300/CREB-binding protein (CBP) transcriptional coactivators and to pRb family members, indicating that this biological activity of E1A is the result of the concomitant perturbation of different cell pathways. Here we show that in the same cells E1A binding to members of the retinoblastoma protein family induces transcriptional down-regulation of the poly(ADP-ribose) polymerase (PARP) gene, coding for a NAD-dependent enzyme stimulated by DNA breaks. Inhibition of PARP expression is accompanied by a decrement of gamma-irradiation-induced apoptosis, which is overridden by reconstitution of wild type levels of PARP. Hence, E1A effects on PARP transcription are central determinant of the apoptotic sensitivity of E1A-expressing keratinocytes. Conversely, E1A binding to only p300/CBP results in an increase in PARP enzyme activity and consequently in cell death susceptibility to irradiation, which is effectively counteracted by the PARP chemical inhibitor 3-aminobenzamide. Therefore, our results identify in the E1A-mediated effects on PARP expression and activity a key molecular event involved in E1A-induced cell sensitization to genotoxic stress.